Attention in the scientific community has avoided exploring LEDs as a viable light source because of their low light intensity (i.e., photon flux) or low photon energy. UV LEDs in the region of 240 nm to 280 nm are now becoming commercially available, although their flux is still limited. Traditional photon-based ion sources have either very high photon energies (e.g., Vacuum UV or VUV lamps and lasers) resulting in direct photo-ionization via Single Photon Ionization (SPI) or via a high intensity or focused laser beam(s), resulting in Multi-Photon Ionization (MPI) or Resonance-Enhanced Multi-Photon Ionization (REMPI). Other UV-laser techniques including, e.g., UV Pulsed-Laser Fragmentation (UV-PLF) have been successfully used to measure photo-fragments of explosive residues using a focused UV laser (MPI). However this approach requires a focused laser beam to permit MPI, and resulting ions are not the parent or close-parent ion fragments, but NO2. With photons that have an energy of between 4 eV to 5 eV per, current UV LEDs lack the ability to directly photo-ionize organic or atmospheric components. In addition, the light from a UV LED has both insufficient energy, for SPI to occur (most organic molecules ionize at >8 eV) and a photon flux that is too low for MPI or REMPI to occur. Photon flux (φ) [(photons/sec)] measured at a distance from an LED is given by Equation [1]:φ=P/(1.602×10−19 J*1240 nm/λ)  [1]
Here, (P) is the measured power (Watts), and (λ) is the wavelength (nm). For currently available UV-LEDS, LEDs with 280 nm and a power of 500 μW generates 7×1014 photons/sec, whereas a UV-LED at 240 nm and 22 μW generates 3×1013 photons/sec. Thus, currently available UV-LEDs are incapable of generating ions via SPI, MPI, or REMPI. Accordingly, new devices and approaches are needed that can take advantage of these low-energy, low-flux light sources to ionize selected organics for analysis.